docs: def unit_system_id(self):
          """
          This is a unique identifier for the unit registry created
          from a FNV hash. It is needed to register a dataset's code
          unit system in the unit system registry.
          """
          if self._unit_system_id is None:
              hash_data = bytearray()
              for k, v in sorted(self.lut.items()):
                  hash_data.extend(k.encode("utf8"))
                  hash_data.extend(repr(v).encode("utf8"))
              m = md5()
              m.update(hash_data)
              self._unit_system_id = str(m.hexdigest())
          return self._unit_system_id